Download Gas Reference Cells Overview

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Optics
Gas Reference Cells Overview
cells sold by Thorlabs, the absorption lines
(wavelengths of light that are absorbed)
are well known. The absolute wavelength
of the absorption lines can be found in
peer reviewed journals and in some cases
online at www.nist.gov. In order to
reproduce the absorption lines reported by
these sources, the temperature and
contents of the reference cell need to be
identical to the cell used in the reported
measurement since changes in
temperature, pressure, and material can
cause shifts and/or broadening of the
absorption lines.
Reference Cell Contents
Free Space Isolators
E-O Devices
Spherical Singlets
Multi-Element
Lenses
Cylindrical Lenses
Aspheric Lenses
Mirrors
Diffusers
& Lens Arrays
Windows
Prisms
Gratings
Polarization Optics
Beamsplitters
Filters
& Attenuators
Gas Cells
Applications:
■ Tunable Laser Calibration
■ Stabilized Frequency Source
■ OSA Calibration
■ Gas Analyzer Calibration
■ Calibration of Wavelength Meter
■ Frequency Standards
Gas reference cells, hereafter referred to as
reference cells, are often used in
applications where the wavelength of light
needs to be accurately determined.
Although spectrometers and optical
spectrum analyzers are built specifically for
this purpose, both need to be accurately
calibrated in order to yield accurate
absolute wavelength measurements. A
reference cell consists of a cylindrical
container (cell) containing a high purity
molecular compound or atomic element.
During the manufacturing of the reference
cell, the temperature and pressure inside
the cell are precisely regulated since these
factors affect the reference cell
performance.
A reference cell can serve as an absolute
wavelength calibration source because the
reference cell will only absorb light at
wavelengths that correspond to the
allowed transitions between energy levels
of the material inside the reference cell.
For the materials offered in the reference
The alkali atoms are located in the first
column of the periodic table. Thorlabs
offers alkali reference cells that contain
sodium, potassium, rubidium, and
cesium. In alkali atom reference cells, the
bulk of the material in the cell will be
coalesced in a solid or liquid form on the
cell walls. However, since the cell was
sealed with vacuum conditions inside the
cell, alkali vapor will be present. The
atomic vapor will absorb light with
wavelengths corresponding to the allowed
transitions between the energy levels of
the atoms. When the reference cell is at
room temperature, the light absorbed will
be principally due to transitions between
the ground state and excited electronic
states of the atom. A reference cell that
contains an alkali atom source is
sometimes referred to as a vapor reference
cell.
Additional vapor reference cells offered by
Thorlabs include cells with water or
iodine as the source material. The iodine
vapor forms the diatomic molecule I2.
Water and iodine absorb light
corresponding to transitions between
rotational and/or vibrational energy levels
in the molecule.
In addition to vapor reference cells, cells
that contain molecules that exist in the
gaseous state at room temperature are
available. When these reference cells are
loaded with the molecular gas and sealed,
the pressure and temperature are precisely
controlled so that it is possible to
reproduce the conditions, and as a result,
the location of the absorption lines
reported in literature. The molecular gas
reference cells sold by Thorlabs include
acetylene, hydrogen cyanide, carbon
monoxide, carbon dioxide, nitrogen
dioxide, methane, ethane, and ethylene.
Each gas has a unique set of absorption
lines corresponding to allowed transitions
between rotational and vibrational energy
levels.
Due to the number of absorption lines in
the NIR region of the electromagnetic
spectrum, molecular gas reference cells
make excellent calibration sources for light
sources and test equipment used in the
Telecom industry. The National Institute
of Standards and Technology (NIST) have
created wavelength reference standards for
the Telecom industry based on Standard
Reference Materials (SRM): 2515 (carbon
monoxide), 2517a (acetylene), and 2519
(hydrogen cyanide).
Reference Cell Containers
Thorlabs’ reference cells are made from
either Pyrex or quartz. Pyrex is an
inexpensive material with good optical
transmission characteristics. Between 350
and 2400nm, the transmission of Pyrex is
over 92% as can be seen on the plot
below. Pyrex is also durable in that it is
less susceptible to breaking or chipping
than quartz. Due to its chemical stability,
100
% Transmission
Optical Systems
Pyrex
80
60
40
20
0
180
1180
2180
Wavelength
3180
Typical saturated absorption experiment using a cesium reference cell that
yields a plot of the Doppler-free Cs absorption line at 852nm.
Cesium Absorption Lines at 852nm
852
www.thorlabs.com
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Optics
Gas Reference Cells Overview… (cont)
a low coefficient of thermal expansion that
makes it resilient to thermal shock. The
thermal properties as well as the welded
100
% Transmission
Pyrex is suitable for holding K, Na, Cs,
and Rb. The standard sized Pyrex cell is
25mm in diameter and 75mm in length.
Quartz is a more expensive material with
excellent optical characteristics. The cell is
composed of a cylindrical quartz tube
with UV fused silica (synthetic fused
quartz) windows on each end. The optical
transmission plot for the UV fused silica
can be seen to the right. Notice that the
transmission range extends much farther
into the UV than it does for Pyrex. In
addition, the UV grade fused silica
windows have an optical flatness of λ/4
for minimal wavefront distortion of the
light passing through the cell. Quartz has
Optical Systems
Fused Silica
80
60
40
20
0
180
1180
2180
Wavelength nm
3180
constructed with windows that have a 2°
wedge. In addition, the window is also
mounted at an angle with respect to the
length of the cell to minimize saturation
and interference effects due to surface
reflections. The angled surfaces prevent
the surface reflection from retracing the
same optical path as the incident light.
Except for potassium and sodium, all
other materials are available in a quartz
reference cell.
Free Space Isolators
E-O Devices
Spherical Singlets
Multi-Element
Lenses
Cylindrical Lenses
Aspheric Lenses
Mirrors
construction of the quartz cell make it
suitable for high-temperature applications
(up to 850°C). The quartz cells are
Diffusers
& Lens Arrays
Windows
Pyrex Reference Cells
Prisms
The quality Pyrex reference cells offered by Thorlabs have optical quality Schott
Borofloat windows and are tested to ensure that the transmission of off resonant light
through the reference cell exceeds 84%. The transmission loss is primarily due to
surface reflections from the windows. The content of the reference cell is also tested by
measuring the absorption spectra of a well known transition using a tunable diode laser.
The following transitions are used for each reference cell: 764nm for potassium, 589nm
for sodium, 780nm for rubidium, and 852nm for cesium.
Gratings
Polarization Optics
Beamsplitters
Filters
& Attenuators
Gas Cells
The standard reference cells are evacuated before the fill material is added and the cell is
sealed so that the fill material is the only material inside the cell. However, the reference
cells can be ordered with the fill material and a buffer gas at a specified pressure. The
noble gases as well as nitrogen are commonly used as buffer gases. The presence of a
buffer gas will cause the central wavelength of the absorption lines to shift and the
width of the absorption lines to broaden.
Specifications
■
■
■
■
■
■
Window Material: Schott
Borofloat Glass
Cell Diameter: 25mm
Fill Stem: <10mm
Window Angle: 0°
Window Type: Flat
Window Thickness: 3mm
Email tech support at [email protected] for custom reference cells.
A tunable laser is used to scan the wavelength of light propagating through the Rb reference
cell. The detector measures the intensity of the transmitted light as a function of wavelenth,
and an oscilloscope records the output of the detector as a function of time. The plot shows
the Doppler broadened absorption spectrum of the D2 line of Rb.
ITEM #
CP25075-RB
CP25075-CS
CP25075-K
CP25075-NA
$
$
$
$
$
434.50
434.50
434.50
434.50
€
£
£
£
£
£
273.70
273.70
273.70
273.70
€
€
€
€
404,10
404,10
404,10
404,10
RMB
¥
¥
¥
¥
4,149.50
4,149.50
4,149.50
4,149.50
PRESSURE
-
>1x10 7 Torr
>1x10 7 Torr
>1x10 7 Torr
>1x10 7 Torr
DESCRIPTION
Ref. Cell, Pyrex Ø25 x 75mm, Rubidium (Rb)
Ref. Cell, Pyrex Ø25 x 75mm, Cesium (Cs)
Ref. Cell, Pyrex Ø25 x 75mm, Potassium (K)
Ref. Cell, Pyrex Ø25 x 75mm, Sodium (Na)
www.thorlabs.com
853
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Optics
Molecular Gas Cells
Optical Systems
The complex absorption line spectrum of the molecular gases offered by Thorlabs
make them excellent fill materials for reference cells used to calibrate the absolute
wavelength of light over a range of wavelengths from 0.5-10µm. The multitude of
absorption lines is a result of the large number of allowed transition between the
various rotational and vibrational energy levels.
Free Space Isolators
E-O Devices
Spherical Singlets
Multi-Element
Lenses
Due to the number of absorption lines in the NIR region of the electromagnetic
spectrum, molecular gas reference cells make excellent calibration sources for the light
sources and test equipment used in the Telecom industry. The National Institute of
Standards and Technology (NIST) have created wavelength reference standards for the
Telecom industry based on Standard Reference Materials (SRM):2515 (carbon
monoxide), 2517a (Acetylene), and 2519 (hydrogen cyanide). In addition to these
gases, Thorlabs also sells molecular gas reference cells with carbon dioxide, nitrogen
dioxide, methane, ethane, and ethylene.
Cylindrical Lenses
Aspheric Lenses
Mirrors
Spectrum Methane Absorption Spectrum
Prisms
Gratings
Polarization Optics
Beamsplitters
Filters
& Attenuators
Gas Cells
Carbon Monoxide Isotope 13 Absorption Spectrum
Transmittance (dB)
Normalized Transmittance
Normalized Transmittance
Acetylene Isotope 13 Absorption
Windows
Normalized Transmittance
Normalized Transmittance
Normalized Transmittance
Custom molecular gas reference cells are
available by request. For example, reference cells
can be created with a mixture of gases at
different pressures to create a calibration source
with absorption lines in more than one spectral
region. A plot is shown below of the absorption
spectrum of a cell that contains a mixture of
acetylene and methane gas. This cell has
absorption lines from 1510-1545nm due to
acetylene as well as absorption lines from 16301670nm due to methane. Other common gas
mixtures used include HCN and carbon
monoxide (12C16O), or HCN and carbon
monoxide (13C16O).
Diffusers
& Lens Arrays
Wavelength (nm)
HCN Absorption Spectrum
ITEM #
CQ09050-CH12
CQ09050-CH13
CQ09075-HCN13
CQ09150-CO12
CQ09150-CO13
CQ09150-CO2
CQ09150-NO2
CQ09075-CH4
CQ09075-C2H6
CQ09075-C2H4
Carbon Monoxide Isotope 12 Absortion Spectrum
$
$
$
$
$
$
$
$
$
$
$
434.50
434.50
434.50
434.50
434.50
434.50
434.50
434.50
434.50
434.50
€
£
£
£
£
£
£
£
£
£
£
£
273.70
273.70
273.70
273.70
273.70
273.70
273.70
273.70
273.70
273.70
€
€
€
€
€
€
€
€
€
€
404,10
404,10
404,10
404,10
404,10
404,10
404,10
404,10
404,10
404,10
RMB
¥
¥
¥
¥
¥
¥
¥
¥
¥
¥
4,149.50
4,149.50
4,149.50
4,149.50
4,149.50
4,149.50
4,149.50
4,149.50
4,149.50
4,149.50
Methane-Acetylene Mixture Absorption Spectrum
PRESSURE
DESCRIPTION
50Torr
50Torr
100Torr
600Torr
600Torr
200Torr
400Torr
200Torr
200Torr
200Torr
Ref. Cell, Quartz Ø9 x 50mm, Acetylene (12C2H2)
Ref. Cell, Quartz Ø9 x 50mm, Acetylene (13C2H2)
Ref. Cell, Quartz Ø9 x 75mm, Hydrogen Cyanide (H13C14N)
Ref. Cell, Quartz Ø9 x 150mm, Carbon Monoxide (12C16O)
Ref. Cell, Quartz Ø9 x 150mm, Carbon Monoxide (13C16O)
Ref. Cell, Quartz Ø9 x 150mm, Carbon Dioxide (CO2)
Ref. Cell, Quartz Ø9 x 150mm, Nitrogen Dioxide (NO2)
Ref. Cell, Quartz Ø9 x 75mm, Methane (CH4)
Ref. Cell, Quartz Ø9 x 75mm, Ethane (C2H6)
Ref. Cell, Quartz Ø9 x 75mm, Ethylene (C2H4)
www.thorlabs.com
855